The hiding of a pigmented coating composition, such as an automotive OEM (original equipment manufacturer) paint or automotive refinish paint, is generally characterized by determining its hiding power, usually by visual observation. The hiding power is the measure of a coating composition's ability to cover a surface opaquely so that an underlying coating, such as that from a primer, cannot be seen in visible light. The absorption and scattering of incident light affects the hiding power of a coating composition. Thus, for example, the compositions of darker colors absorb more intensely than those of lighter colors and hence such compositions have greater hiding power than those of lighter colors.
Several methods for determining the hiding power of a coating composition are known in the coatings art. One such method (ASTM D 6762-02a) supplied by ASTM International, West Conshohocken, Pa.) consists of applying a monotonic, i.e., wedge shaped, layer of a coating composition over a test hiding pattern affixed to a hiding panel. After the applied layer cures or dries into a coating, then visually observing the coating under light at an angle perpendicular to the coating to determine a position on the coating where the test pattern is barely or no longer visible. The aforementioned test pattern is typically in the form of two abutting white and black stripes and the monotonic layer of increasing gradient is thinnest at one end of the test pattern and thickest at the other end of the test pattern. The monotonic coating is typically produced by progressively increasing the number of spraying passes over one end of the panel to the other. Typically, on one end of the panel, the test pattern is clearly visible through the coating (non-hiding end) whereas the test pattern is not visible at other end (hiding end). The hiding power of a coating composition is the lowest coating thickness at which hiding occurs. That would be the coating thickness at which the coating, such as paint, should be applied. Anything less will not be adequate and anything more would be a waste. The process for measuring the hiding power of a coating composition is currently done by technicians who visually examine the hiding panel and mark the position where the hiding pattern is no longer visually discernable. A film thickness gage is then used to measure the coating thickness at this position on the panel and recorded as the hiding film thickness for that particular coating. This approach is prone to significant errors because of visual subjectivity, due to variations in lighting used for observing the panel and observing geometry variation. Furthermore, the film thickness measured by a gage is very sensitive to the technique of usage and adds another significant source of error.
Some prior art references describe instruments for measuring color difference between two areas and when this difference goes below a pre-determined value, the coating thickness at that point would represent the hiding thickness of that coating composition. However, using the same pre-determined value to represent hiding for different colors and finishes does not produce the best results. For finishes that have high sparkle from metallic flakes, hiding occurs even with relatively high color difference whereas for solid colors, such as certain whites and yellows, hiding only occurs at extremely low values of color difference. Thus, using a single predetermined value does not produce accurate results. Furthermore, color and film thickness measurements on typically prepared hiding panels can have various random errors in them. Air bubbles under the hiding test pattern sticker, smudges and scratches, etc., can result in incorrect readings of color intensities. Similarly, the coating thickness measurements at any one point can have errors. Thus, most of the known methods fail since they have the aforedescribed errors. Thus, a need exits for a method and hiding measurement apparatus that is practical in that it is adaptive and error correcting and it accurately determines the hiding of various colors and coating compositions, including those that contain flakes, such as metallic, pearlescent, and mineral flakes
The patent publication GB 1404 636 describes a system and a method for determining the hiding power of paints. According to the patent publication, a layer of paint is applied on a substrate with black and white regions. The substrate is illuminated with light, and the light reflected from the black and white regions of the substrate is captured by photoelectric cells. Potential difference measured by using photoelectric cells is considered to be proportional to the difference in reflective value of the black and white regions. Hiding power is presumed to be a function of the difference in the reflectance over the black and white regions. However, the patent publication only mentions a system and method for determining hiding power of paint. It is not directed to determining the hiding thickness at which the hiding occurs. Therefore, a need still exists for more accurately and consistently determining the hiding thickness of coating compositions, even on wet painted substrates.